This invention relates in general to respirators and in particular to a new and useful method and apparatus for controlling the flow of breathing gas to a patient and for creating a measuring interval during the respiration in which various breathing gas characteristics are determined.
Substantial problems arise in the monitoring of an artificial respiration with a quick and accurate determination of breathing gas values and ventilation parameters, especially if a high frequency pulse method where a jet nozzle (High Frequency Jet Ventilation or HFJV) is used.
German Pat. No. 20 47 659 discloses a ventilation system wherein the breathing gas source in HFJV operation produces high-pressure gas-pulses having a pulse recurrence frequency in excess of 600 per minute, and alternately pulse trains and pauses therebetween are provided. The pauses are intended as intervals for examining gas samples. The gas samples are analyzed in a respective device and depending on the result, the control of the ventilation can be adjusted to obtain the best respiration pattern.
The CO.sub.2 concentration, i.e. CO.sub.2 content in the exhaled air, which is measured at the end of an expiration phase to determine the partial pressure of CO.sub.2 in the blood, is a generally acknowledged important control variable in conventional ventilation (Intermittent Positive Pressure Ventilation or IPPV) without a pulse control. While in a conventional ventilation, the determination of the CO.sub.2 content of the exhaled air is not particularly difficult, during high-frequency ventilation, the CO.sub.2 determination is not so easy. The exhaled gas is strongly diluted by the continuous flow of flush gas, and because of the minimum volumina per stroke during the high-frequency ventilation, which are on the order of magnitude of the dead space of the breathing gas supply device the CO.sub.2 concentration in the exhaled gas does not correspond to that of the alveolar region.
Another problem in high-frequency ventilation of a patient is the determination of the constant basic pressure established in the lungs, the so called "lung-internal PEEP". At high ventilation frequencies, about in the range of 600 per minute, such as usual in this kind of ventilation, the so called "air trapping" builds up a pressure in the lungs, which does not decay at the end of the short expiration phases. With methods hitherto known, this internal PEEP (Positive End Expiratory Pressure) cannot be measured with satisfactory accuracy.
A prior art qualitative measurement utilizes thorax bloating for inferring therefrom the pressure built up in the lungs. To this end, the thorax impedance can be determined in a known manner through electrodes applied to the chest, or strain bands placed on the chest, extending and thus providing resistance values to be measured. Such measurements furnish only coarse information on the lung-internal PEEP and the fixing of measuring elements or bands may be difficult, particularly with a traumatic chest.